In recent years, with rapid growth in the demand for mobile voice service, GSM network will enter into a great development. However, limited frequency resources become increasingly difficult to meet the needs during people's calls, especially in densely populated cities. With the aging of existing GSM network equipments, it is more urgent to expand existing GSM network.
Multiple User Reuse One Slot (MUROS) technology is mainly used in 3GPP GSM/EDGE Radio Access Network (GERAN). Study Item stage will be discussed from GERAN#36 meeting, aimed at enhancing user capacity of GSM system, and helping network operators to alleviate the pressure. In Work Item stage, it is called VAMOS (Voice Services over Adaptive Multi-user Channels on One Slot).
MUROS needs to further enhance the voice capacity on the basis of reusing existing network equipments and wireless resources. In feasibility study stage, candidate MUROS solutions mainly focus on two or more users reusing the same time slot on the premise that call quality is not compromised. At present, main consideration is given to improving the system's voice capacity by two times, that two users reuse one slot. What is affected is full-rate and half-rate voice channels, TCH/FS, TCH/HS, TCH/EFS, TCH/AFS, TCH/AHS, and TCH/WFS, as well as associated control channels, such as slow associated control channel (SACCH, including full-rate SACCH/TF and half-rate SACCH/TH, where SACCH described below in this invention also includes full-rate and half-rate) and fast associated control channel (FACCH, including full-rate FACCH/F and half-rate FACCH/H, where FACCH described below in this invention also includes full-rate and half-rate).
Existing MUROS solutions mainly include the following four categories:
1) Co-TCH Program
Downlink: linear combined two-way Gaussian minimum shift keying (GMSK) baseband modulation signals (two-way phase difference Pi/2), are sent out upon RF modulation and power amplification.
Uplink: each mobile station uses GMSK modulation respectively, and adopts different training sequence (TSC), to separate out two-way reusing user signals at base station by joint detection methods.
2) Orthogonal Subchannel (OSC) Program
Downlink: a quadri-phase shift keying (QPSK) is used to transmit two-way user signals.
Uplink: each mobile station uses GMSK modulation respectively, and use different training sequences, to separate two-way reused signals at base station by interference elimination methods.
3) Adaptive Symbol Constellation Mapping (ASC) Program
Downlink: alpha-QPSK program, with phase difference alpha between the same channel and orthogonal channel.
Uplink: each mobile station uses GMSK modulation respectively, and adopts different training sequences, to demodulate two-way reused signal at base station with multi-user-multiple-input multiple-output (MU-MIMO) receivers.
4) High-order Modulation (HOM) Program
Downlink: each user is separately numbered, and GMSK, QPSK, 8PSK, or 16QAM programs can be used upon rate matching to reuse data of 1-4 users in the same time-frequency resources.
Uplink: up to two users share the same time-frequency resources, using GMSK or QPSK modulation, and distinguishing the users through the training sequence.
At present, in feasibility study of MUROS, main consideration is given to the above-mentioned four programs, allowing two users to share the same time-frequency resources to improve the system capacity by two times. However, which program to be used in 3GPP GERAN is still under discussion. No matter whichever program, it can be regarded as there are two sub-channels respectively, allocated to a user, forming a user group, or known as a pair of users.
FIG. 1 shows frame structure of GSM system. GSM system uses Time Division Multiple Access (TDMA), in which each TDMA frame is divided into eight time slots, numbered from 0, 1, . . . , 7, regardless of whether frames or time slots are non-overlapping. Time slot is basic wireless resource unit in GSM system. On the principle of certain time slot distribution, each mobile station in each frame can only sends signals to base station in designated time slots. When it meets timing and synchronization conditions, base station can receive separately signals from each mobile station in each time slot without mixing and interference mutually. Meanwhile, signals sent from base station to multiple mobile stations are arranged in order, and transmitted in scheduled time slot. If only mobile stations receive signals in specified time slots, they will be able to distinguish signals received from combined signals.
FIG. 2 shows MUROS frame structure. In MUROS system, on the same carrier frequency, there are two users reusing the same slot; in the downlink, users are identified through I (In-phase) and Q (Quadrature) phase information, while it may be considered as a physical channel is divided into two sub-channels, respectively allocated to a user. FIG. 2 shows a typical two users reusing, each of which occupies a sub-channel. Of course, two sub-channels may also be allocated to a user at the same time.
In GSM system, discontinuous transmission (DTX) technology is used; when voice codec detects voice interval, it will not send during interval period.
The role of DTX:
1. To use DTX to reduce interference level in the system, and improve the effectiveness of the system.
2. Due to the use of DTX transmitter, total time for transmission is lowered, to reduce power consumption and extend service life of MS battery.
In order to achieve DTX, one has to use voice activity detection (VAD), that once the speech pauses, a reminder is given.
In MUROS downlink, as Legacy user and MUROS users occupy the same time-frequency resources during transmission, such two users are called a pair of users. But when one of Legacy users enters into DTX stage, or ends its calls, or switch to other neighbouring districts, such two users will not be able to maintain the state of pairing, and this user will be converted to non-MUROS state or pending for pairing states.
In this case, at present, a good program is to use diversity transmission technology on remaining MUROS users in this user group, that is remaining users send the same information through downlink subchannels.
FIG. 3 shows frame structure in MUROS system during diversity transmission. As shown in the figure, when one of users come into DTX state, or ends its calls, another user may send the same data in two-way subchannels to improve transmission reliability.
At transmitting end, the advantages of using diversity transmission are as follows:
1) It may improve channel quality, in particular, improve control channel SACCH and FACCH channel conditions. In bad channel conditions, it is essential to improve the reliability of control channel;
2) It may easily transfer into pairing state, only if one of sub-channels is allocated to users that newly access to.
At receiving end, a user will need to receive information from two channels, carry out autocorrelated operations over training sequence that it receive and its training sequence, and determine whether the information from the channel is sent to itself. If one adopts diversity transmission, the users may combine the information from such two channels upon demodulation in order to obtain diversity gain; otherwise, the users only demodulate information from sub-channels allocated to themselves initially, and do not obtain diversity gain at this time.